Use strings.Builder for gRPC percent methods

[emcfarlane]

Both grpcPercentEncode and grpcPercentDecode need to allocate a string and can use strings.Builder to own the allocation. Small optimization with Grow to give a hint to the size.

goos: darwin
goarch: arm64
pkg: github.com/bufbuild/connect-go
                      │ current.txt  │                new.txt                │
                      │    sec/op    │    sec/op     vs base                 │
GRPCPercentEncoding-8   408.6n ± ∞ ¹   371.7n ± ∞ ¹        ~ (p=1.000 n=1) ²
GRPCPercentDecoding-8   79.59n ± ∞ ¹   65.24n ± ∞ ¹        ~ (p=1.000 n=1) ²
geomean                 180.3n         155.7n        -13.65%
¹ need >= 6 samples for confidence interval at level 0.95
² need >= 4 samples to detect a difference at alpha level 0.05

                      │ current.txt │               new.txt                │
                      │    B/op     │    B/op      vs base                 │
GRPCPercentEncoding-8   51.00 ± ∞ ¹   64.00 ± ∞ ¹        ~ (p=1.000 n=1) ²
GRPCPercentDecoding-8   16.00 ± ∞ ¹   32.00 ± ∞ ¹        ~ (p=1.000 n=1) ²
geomean                 28.57         45.25        +58.42%
¹ need >= 6 samples for confidence interval at level 0.95
² need >= 4 samples to detect a difference at alpha level 0.05

                      │ current.txt │               new.txt                │
                      │  allocs/op  │  allocs/op   vs base                 │
GRPCPercentEncoding-8   7.000 ± ∞ ¹   2.000 ± ∞ ¹        ~ (p=1.000 n=1) ²
GRPCPercentDecoding-8   1.000 ± ∞ ¹   1.000 ± ∞ ¹        ~ (p=1.000 n=1) ³
geomean                 2.646         1.414        -46.55%
¹ need >= 6 samples for confidence interval at level 0.95
² need >= 4 samples to detect a difference at alpha level 0.05
³ all samples are equal

[akshayjshah]

Works for me :)