Add support for GCP extensions

[igor-kupczynski]

The support is modeled on azure.go under tlvparse. The GCP spec is similar but
encodes uint64. There's no subtype. Since zero value of uint64 is technically
a valid header value, I've opted for an extra boolean to indicate if it was
found (same as with Azure).

Given that we already have AWS, and Azure in tlvparse I thought it will be a good idea to add the GCP support as well.

We use it with GCP in our pre-prod env and all seems to be working well.

Let me know if you think it makes sense to have this with the upstream lib.

[coveralls]

Coverage remained the same at 94.353% when pulling 9f25ec7 on igor-kupczynski:gcp-tlv-parser into 3aa7ea9 on pires:main.

[igor-kupczynski]

I've noticed a naming inconsistency and force-pushed a fixed version. Hope that OK -- my thinking was that a simple rename doesn't add a lot to the history.

[pires]

The only thing I'd like to see here and maybe it's something for the future is to provide some structure, such as func names tied to PP2 type, eg PP2GCP.ExtractPSCConnectionID, or maybe have tlvparse/{aws,azure,gcp} packages so folks don't need to read the code to know which functions are available to them for a particular type.

That said, thank you very much for your contributions! 🙇🏻

[igor-kupczynski]

Nice, thanks for merging :) and thanks for maintaining the library.

func names tied to PP2 type, eg PP2GCP.ExtractPSCConnectionID, or maybe have tlvparse/{aws,azure,gcp}

Agree, it would be nice to improve the consistency here. I'll think about it for a bit.

[pires]

Released in https://github.com/pires/go-proxyproto/releases/tag/v0.6.1