Support DECIMAL literals

[big-andy-coates]

Similar to #3591 but for DECIMALs.

We need a way for users to construct decimals. Currently, our SQL parsing rules handle DECIMAL creation:

DECIMAL_VALUE
    : DIGIT+ '.' DIGIT*
    | '.' DIGIT+
    | DIGIT+ ('.' DIGIT*)? EXPONENT
    | '.' DIGIT+ EXPONENT
    ;

However, this is converted to a DoubleLiteral in ParserUtil:

public static DoubleLiteral parseDecimalLiteral(final DecimalLiteralContext context) {
   ...
}

Given we don't support implicit conversion of DOUBLE -> DECIMAL, and any such conversion would be potentially lossy anyway, it means we don't have any way for users to create DECIMAL literals to use in INSERT VALUES or indeed to create DECIMALs to use in any other part of our SQL dialect.

We should look to change our current decimal parsing to return a DECIMAL literal, though doing so would be a breaking change without careful management. We can maybe look to add a compatibility breaking config to decide how decimals should be parsed, and then look to remove this option in the next major release.

[agavra]

Perhaps this isn't desired behavior, but you can currently use INSERT INTO with a decimal if you represent it as a string (e.g. INSERT INTO topic (decimalCol) VALUES ("1.12345"))

[big-andy-coates]

We should consider make a breaking change in the next major version so that 12.4555 is a decimal literal, which can implicitly be converted to a double, and not a double, which a) might not be able to represent the number exactly and b) doesn't support implicit casting to decimal.

Marking for 6.0 release.

[big-andy-coates]

When we switch to proper decimal support we should remove the magic around string -> decimal coercion. e.g. in INSERT VALUES functionality, and in ExpectedRecordComparator.java