Allow raw binary access via GetFieldValue<byte[]> and GetBytes

[roji]

#3152 allowed calling NpgsqlDataReader.GetStream on any data type; this allows e.g. getting a raw string representation for decoding outside of Npgsql, for efficient JSON decoding.

We could allow the same for NpgsqlDataReader.GetFieldValue<byte[]> and NpgsqlDataReader.GetBytes(), which could be useful in certain short value scenarios. We should probably wait to see if someone cares first.

[Emill]

I could think that these methods should be called GetRawData or something, to avoid people accidentally getting a byte[] on a column instead of an exception, in case they for example used the wrong column ordinal.

[roji]

@Emill I see what you mean, though:

• We already allow this for GetStream, which seems to be the same case (i.e. no GetRawStream)
• GetRawData would be Npgsql-specific (whereas GetFieldValue can be used from DbDataReader)
• NpgsqlDataReader is already a type with many methods, so we shouldn't add more too lightly.
• It could be claimed that if people accidentally use the raw ordinal, they'd know pretty quickly as the data they get back would not be expected. So IMHO the actual potential for bugs probably doesn't justify adding a new API.

[NinoFloris]

GetBytes is supported in #5123, adding general GetFieldValue<byte[]> support is probably best left out as it'd be another type check on the hot path.