feat(taskworker) Add signature based authentication to RPC calls

[markstory]

Longer term we may be able to use service mesh authentication, but the requirements for that incur additional infrastructure complexity. This level of authentication will prevent untrusted clients from fetching and updating tasks.

Refs getsentry/taskbroker#57

[evanh]

If this is an internal-only service why do we need authentication at all?

[codecov]

Codecov Report

Attention: Patch coverage is 96.77419% with 2 lines in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
tests/sentry/taskworker/test_client.py	93.75%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #85533       +/-   ##
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+ Coverage   42.27%   87.92%   +45.64%     
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  Files        9615     9646       +31     
  Lines      544191   546071     +1880     
  Branches    21242    21242               
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+ Hits       230082   480132   +250050     
+ Misses     313800    65630   -248170     
  Partials      309      309               

[markstory]

If this is an internal-only service why do we need authentication at all?

Should adversaries get access to an internal network, I'd like to avoid any possibilities of ssrf, or malicious requests being made unless the attacker also compromises application containers.