This commit adds a `MarshalText` implementation
to the `crypto.Context` type.
The `MarshalText` implementation replaces the
`WriteTo` and `AppendTo` implementation.
It is slightly slower than the `AppendTo` implementation
```
goos: darwin
goarch: arm64
pkg: github.com/minio/minio/cmd/crypto
BenchmarkContext_AppendTo/0-elems-8 381475698 2.892 ns/op 0 B/op 0 allocs/op
BenchmarkContext_AppendTo/1-elems-8 17945088 67.54 ns/op 0 B/op 0 allocs/op
BenchmarkContext_AppendTo/3-elems-8 5431770 221.2 ns/op 72 B/op 2 allocs/op
BenchmarkContext_AppendTo/4-elems-8 3430684 346.7 ns/op 88 B/op 2 allocs/op
```
vs.
```
BenchmarkContext/0-elems-8 135819834 8.658 ns/op 2 B/op 1 allocs/op
BenchmarkContext/1-elems-8 13326243 89.20 ns/op 128 B/op 1 allocs/op
BenchmarkContext/3-elems-8 4935301 243.1 ns/op 200 B/op 3 allocs/op
BenchmarkContext/4-elems-8 2792142 428.2 ns/op 504 B/op 4 allocs/op
goos: darwin
```
However, the `AppendTo` benchmark used a pre-allocated buffer. While
this improves its performance it does not match the actual usage of
`crypto.Context` which is passed to a `KMS` and always encoded into
a newly allocated buffer.
Therefore, this change seems acceptable since it should not impact the
actual performance but reduces the overall code for Context marshaling.
7b3719c17b