teach vcrpy our proxy v2 request/response semantics

[cfm]

Tests need fixing, they're currently using vcr. We either need to reimplement vcr for our mock format or mock the proxying to use requests and then keep using vcr.

Originally posted by @legoktm in #1718 (comment)

[cfm]

We know that requests look like:

securedrop-client/client/securedrop_client/sdk/__init__.py

Lines 160 to 169 in f8c1f54

	if method == "POST" and body:
	data["body"] = body
	if headers:
	data["headers"] = headers
	if timeout:
	data["timeout"] = timeout
	data_str = json.dumps(data).encode()

And responses look like either of:

securedrop-client/client/securedrop_client/sdk/__init__.py

Lines 74 to 85 in f8c1f54

	@dataclass(frozen=True)
	class StreamedResponse:
	contents: bytes
	sha256sum: str
	filename: str
	@dataclass(frozen=True)
	class JSONResponse:
	data: dict
	status: int
	headers: Dict[str, str]

So, working backwards:

1. we have an obvious custom persister; and
2. we need to hook vcrpy itself into sdk._send_json_request().

[cfm]

This is well underway in proxy-rusting, where VCR.py is able to de/serialize our new {JSON,Streamed}Responses without any further intervention on our part. The interesting bits are f3a5c5a...4763792.

Next up is to deduplicate test_apiproxy.py and test_api.py, which are redundant now that we intend for the SDK to always use the proxy.

[cfm]

This is implemented in f3a5c5a...b82720b. The diffstat looks horrific (99 files changed, 14113 insertions(+), 5595 deletions(-) because a bunch of one-line JSON fixtures are now multi-line YAML fixtures. There are still fewer of them—50% fewer, in fact, in a single converged test path instead of two.

Commits are marked [WIP] because there are two subtleties in the VCR.py instrumentation of securedrop_client.sdk.API._send_json_request() that I should document.

[cfm]

This will be reviewed as part of #1718.