[Drilldowns] URL Drilldown template helpers

[Dosant]

Part of #55324

Describe the feature:

Currently URL drilldown templating provides 3 custom helpers (rison, json, date) is addition to build-in handlebars helpers.
We should extend list of helpers with helpers for string manipulation, equity checks and others.
In addition to that, we should expose an extension point so plugin developers could extend list of helpers.

Preliminary list of must have helpers:

UPPER
LOWER
LEFT
MID
RIGHT
CONCAT
TRIM
LTRIM
RTRIM

• Take a look at for inspiration: https://github.com/helpers/handlebars-helpers

Describe a specific use case for the feature:

Example use case: #75450 (comment)

Registering custom helpers tracked here: #78404

cc @elastic-jb @streamich

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[Dosant]

• Consider number formatting: [discuss][Drilldowns][URL drilldown] Expose "event.row" in VALUE_CLICK_TRIGGER #79157 (comment)