feat: add Decoder.SetMaxFieldLength

[wojas]

Add a Decoder.SetMaxFieldLength method to allow overriding the default field limit of 2GB, as described in issue #207.

This PR also adds a test for the feature and ensures that nothing changes for existing code.

benchstat

goos: darwin
goarch: arm64
pkg: github.com/CrowdStrike/csproto
cpu: Apple M1 Pro
                    │   old.txt   │              new.txt               │
                    │   sec/op    │   sec/op     vs base               │
EncodeString-10       5.589n ± 0%   5.564n ± 0%       ~ (p=0.218 n=50)
SafeDecodeString-10   17.96n ± 1%   17.69n ± 0%  -1.50% (p=0.000 n=50)
FastDecodeString-10   8.194n ± 0%   8.104n ± 0%  -1.09% (p=0.000 n=50)
DecodeFixed32-10      5.369n ± 1%   5.460n ± 1%  +1.70% (p=0.000 n=50)
DecodeFixed64-10      5.394n ± 1%   5.478n ± 1%  +1.56% (p=0.000 n=50)
geomean               7.505n        7.508n       +0.03%