[Go SDK] Add the standard millis_instant and decimal logical types

[Eliaaazzz]

Part of #39684, and the Go side of #19817. Depends on #40154 (the first three commits); review the last commit.

Both types have a wire encoding that differs from the row encoding of their representation type, so they use schema.RegisterLogicalTypeCoder:

• schema.MillisInstant (beam:logical_type:millis_instant:v1), a time.Time with millisecond precision. The representation is INT64, but Java (InstantCoder) and Python (TimestampCoder, special cased in row_coder.py) encode it as 8 big endian bytes shifted by the minimum int64, so the byte order matches the chronological order. The Go encoding uses the same shift as coder.EncodeEventTime. Encoding a value with sub millisecond precision returns an error.
• schema.Decimal (beam:logical_type:decimal:v1), an arbitrary precision decimal with Unscaled *big.Int and Scale int32. The representation is BYTES, encoded as Java's BigDecimalCoder and Python's DecimalCoder do: the scale as an unsigned 32 bit varint (the INT32 varint form, so negative scales such as 1E+3 round trip with Java) followed by the length prefixed two's complement bytes of the unscaled value. ParseDecimal and String convert to and from plain notation.

The millis_instant and decimal cases of standard_coders.yaml now run in the Go regression test.

MillisInstant and Decimal are registered with runtime.RegisterType like the other standard types.

Tests: byte level row encoding against the yaml examples for both types (including the negative decimal and the shifted millis value), two's complement conversion at the byte boundaries, decimal parsing and formatting, and the sub millisecond error.

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[Eliaaazzz]

Closing this for now to keep the series at three open PRs while #40152, #40153 and #40154 are reviewed. The branch stays in my fork.

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R: @shunping for label go.

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[shunping]

Thanks for the contribution, @Eliaaazzz!

Since the prior PRs in this series are still being reviewed, I'd prefer to wait until those go through the pipeline first to avoid any cascading changes you might have to make here.

[Eliaaazzz]

Thanks, that makes sense. I moved this PR and #40156 to draft until #40152 to #40154 are merged, and I will rebase them then.