Standardize runtime field emit methods

[nik9000]

This standardizes that name for all of the "emit value" methods used by
runtime field scripts to emitValue. To make dates possible this adds a
a toEpochMilli method to convert from TemporalAccessor into millis
since epoch.

[elasticmachine]

Pinging @elastic/es-search (:Search/Search)

[javanna]

LGTM, only one question: with this every context supports one return type, and some types require conversions through specific function calls to adapt values to the required return type? I guess with this I am double checking that emitValue always and only accept one argument type per context.

[nik9000]

with this every context supports one return type, and some types require conversions through specific function calls to adapt values to the required return type? I guess with this I am double checking that emitValue always and only accept one argument type per context.

All correct. Painless itself doesn't have support for arguments with different types so we just pick a "native" type for each.