signer: Add price ceilings

[j0sh]

Add optional maxPrice fields to payment requests and auth webhook responses. This works in conjunction with the signer-configured max; if all three are present, the minimum is effectively used.

Also cap the session price at the initial price, since orchestrators should also fix the price at session start, so there is no need to accommodate ~2x drift within ValidatePrice. In practice this should not lead to a behavior change.

[rickstaa]

@j0sh valuable addition which makes sense. Just left some minor comments but can be merged regardless.

[j0sh]

The x264 git repo went offline which broke tests / CI, so had to update LPMS (livepeer/lpms#451) and then in f4dde49

[codecov]

Codecov Report

❌ Patch coverage is 94.54545% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 35.51633%. Comparing base (cc49228) to head (77fae56).

Files with missing lines	Patch %	Lines
server/remote_signer.go	94.54545%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #4014         +/-   ##
===================================================
+ Coverage   35.46423%   35.51633%   +0.05210%     
===================================================
  Files            174         174                 
  Lines          45161       45213         +52     
===================================================
+ Hits           16016       16058         +42     
- Misses         27871       27880          +9     
- Partials        1274        1275          +1     

Files with missing lines	Coverage Δ
server/remote_signer.go	67.07566% <94.54545%> (+1.62944%)	⬆️

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Files with missing lines	Coverage Δ
server/remote_signer.go	67.07566% <94.54545%> (+1.62944%)	⬆️

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[rickstaa]

The x264 git repo went offline which broke tests / CI, so had to update LPMS (livepeer/lpms#451) and then in f4dde49

Thanks!