implement NOW() siblings

[jycor]

This PR has our behavior surrounding NOW() functions more closely match MySQL.

Changes:

• Added NOW() synonyms to registry
• Have CURRENT_TIMESTAMP(), LOCALTIME(), LOCALTIMESTAMP() all just call NOW()
• Support parsing synonyms in DEFAULT and ON UPDATE expressions
• Fixed SHOW CREATE TABLE to print CURRENT_TIMESTAMP for NOW() and synonyms

Companion PR: dolthub/vitess#296
Fixes:

• support current_timestamp synonyms dolt#7129
• current_timestamp() and now() do not return the same values dolt#6058

[max-hoffman]

LGTM, the biggest thing I'm noticing is that we could frontload some of the precision resolving. Not blocking though

[max-hoffman]

Lot of overlap with the default resolving, can we push all of this into planbuilder when resolving now functions? It might be easy to resolve as an integer type upfront also, one less expression to track through analysis

[jycor]

It looks like there are other functions, like CURRENT_TIME and UTC_TIMESTAMP, that should be using similar logic. Will do this refactor in a follow up PR.