go/runtime/host: Improve common.GetQuotePolicy function

[martintomazic]

Trivial, semantics should be preserved.

Tackles #6474 (comment).

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[martintomazic]

Things can get further simplified now:

1. teeState interface no longer makes sense and mock flag should instead be nested with ifs inside the ecdsa like is done elsewhere for the mock flag. Also existing name is confusing, Attester would probably be easier to read, but probably no longer needed anyways.
2. Get rid of the common package nested inside sgx since it is sgx agnostic and common packages are a smell. E.g. move one level up to attestation package:

runtime/host/attestation/policy.go
  QuotePolicyProvider

runtime/host/attestation/??.go
  UpdateRuntimeQuote
  EndorseCapabilityTEE

runtime/host/attestation/worker.go
  AttestationWorker

runtime/host/attestation/metrics.go
  InitMetrics
  UpdateAttestationMetrics

[codecov]

Codecov Report

❌ Patch coverage is 64.28571% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.83%. Comparing base (18756c5) to head (4b0af05).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
go/runtime/host/tdx/qemu.go	40.00%	9 Missing ⚠️
go/runtime/host/provisioner/provisioner.go	72.22%	3 Missing and 2 partials ⚠️
go/runtime/host/sgx/state.go	57.14%	2 Missing and 1 partial ⚠️
go/runtime/host/sgx/ecdsa.go	75.00%	1 Missing and 1 partial ⚠️
go/runtime/host/sgx/mock.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6531      +/-   ##
==========================================
+ Coverage   63.75%   63.83%   +0.07%     
==========================================
  Files         698      697       -1     
  Lines       68293    68166     -127     
==========================================
- Hits        43542    43511      -31     
+ Misses      19757    19675      -82     
+ Partials     4994     4980      -14     

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[peternose]

further simplified now

I would do this in another PR, if needed.