Merge bitcoin/bitcoin#27101: Support JSON-RPC 2.0 when requested by client

cbc6c440e3811d342fa570713702900b3e3e75b9 doc: add comments and release-notes for JSON-RPC 2.0 (Matthew Zipkin)
e7ee80dcf2b68684eae96070875ea13a60e3e7b0 rpc: JSON-RPC 2.0 should not respond to "notifications" (Matthew Zipkin)
bf1a1f1662427fbf1a43bb951364eface469bdb7 rpc: Avoid returning HTTP errors for JSON-RPC 2.0 requests (Matthew Zipkin)
466b90562f4785de74b548f7c4a256069e2aaf43 rpc: Add "jsonrpc" field and drop null "result"/"error" fields (Matthew Zipkin)
2ca1460ae3a7217eaa8c5972515bf622bedadfce rpc: identify JSON-RPC 2.0 requests (Matthew Zipkin)
a64a2b77e09bff784a2635ba19ff4aa6582bb5a5 rpc: refactor single/batch requests (Matthew Zipkin)
df6e3756d6feaf1856e7886820b70874209fd90b rpc: Avoid copies in JSONRPCReplyObj() (Matthew Zipkin)
09416f9ec445e4d6bb277400758083b0b4e8b174 test: cover JSONRPC 2.0 requests, batches, and notifications (Matthew Zipkin)
4202c170da37a3203e05a9f39f303d7df19b6d81 test: refactor interface_rpc.py (Matthew Zipkin)

Pull request description:

  Closes https://github.com/bitcoin/bitcoin/issues/2960

  Bitcoin Core's JSONRPC server behaves with a special blend of 1.0, 1.1 and 2.0 behaviors. This introduces compliance issues with more strict clients. There are the major misbehaviors that I found:
  - returning non-200 HTTP codes for RPC errors like "Method not found" (this is not a server error or an HTTP error)
  - returning both `"error"` and `"result"` fields together in a response object.
  - different error-handling behavior for single and batched RPC requests (batches contain errors in the response but single requests will actually throw HTTP errors)

  https://github.com/bitcoin/bitcoin/pull/15495 added regression tests after a discussion in https://github.com/bitcoin/bitcoin/pull/15381 to kinda lock in our RPC behavior to preserve backwards compatibility.

  https://github.com/bitcoin/bitcoin/pull/12435 was an attempt to allow strict 2.0 compliance behind a flag, but was abandoned.

  The approach in this PR is not strict and preserves backwards compatibility in a familiar bitcoin-y way: all old behavior is preserved, but new rules are applied to clients that opt in. One of the rules in the [JSON RPC 2.0 spec](https://www.jsonrpc.org/specification#request_object) is that the kv pair `"jsonrpc": "2.0"` must be present in the request. Well, let's just use that to trigger strict 2.0 behavior! When that kv pair is included in a request object, the [response will adhere to strict JSON-RPC 2.0 rules](https://www.jsonrpc.org/specification#response_object), essentially:

  - always return HTTP 200 "OK" unless there really is a server error or malformed request
  - either return `"error"` OR `"result"` but never both
  - same behavior for single and batch requests

  If this is merged next steps can be:

  - Refactor bitcoin-cli to always use strict 2.0
  - Refactor the python test framework to always use strict 2.0 for everything
  - Begin deprecation process for 1.0/1.1 behavior (?)

  If we can one day remove the old 1.0/1.1 behavior we can clean up the rpc code quite a bit.

ACKs for top commit:
  cbergqvist:
    re ACK cbc6c440e3811d342fa570713702900b3e3e75b9
  ryanofsky:
    Code review ACK cbc6c440e3811d342fa570713702900b3e3e75b9. Just suggested changes since the last review: changing uncaught exception error code from PARSE_ERROR to MISC_ERROR, renaming a few things, and adding comments.
  tdb3:
    re ACK for cbc6c440e3811d342fa570713702900b3e3e75b9

Tree-SHA512: 0b702ed32368b34b29ad570d090951a7aeb56e3b0f2baf745bd32fdc58ef68fee6b0b8fad901f1ca42573ed714b150303829cddad4a34ca7ad847350feeedb36

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