crypto: simplify Context encoding (#11812)

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.

cmd/crypto/error.go

@@ -81,8 +81,6 @@ var (
 
 	errInvalidInternalIV            = Errorf("The internal encryption IV is malformed")
 	errInvalidInternalSealAlgorithm = Errorf("The internal seal algorithm is invalid and not supported")
-
-	errMissingUpdatedKey = Errorf("The key update returned no error but also no sealed key")
 )
 
 var (