SQL: Allow whitespaces in escape patterns

[matriv]

Previously, we supported only the format {fn <FUNCTION_NAME>()}
but other DBs like MSSQL, DB2, MariaDB/MySQL alos allow whitespaces
between { and fn. Furhermore, also some applications - like PowerBI -
generate escape sequences with spaces: select { fn name(params) } etc.

Add support for white spaces between { and the escape pattern definition
like fn, ts, d, guid etc.

Closes: #47401

[elasticmachine]

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

[matriv]

I'm considering to backport it to earlier versions too, more like a fix.
What would you think?

[matriv]

There is another possibility to refactor the grammar like this: https://github.com/matriv/elasticsearch/compare/impl-47401...matriv:refactor-grammar?expand=1#diff-14b1f1a74854b9993f46da9467f6abcb so that we avoid the explicit (WS)*.

Personally I prefer the solution as is currently in the PR, which distinguishes the LIMIT from the LIMIT_ESC and the ESCAPE from the ESCAPE_ESC.

[astefan]

LGTM

[bpintea]

LGTM

[costin]

LGTM
For the record, the JDBC spec mandates that there should be no whitespace in the header (e.g. {fn NOT { fn ).

[costin]

LGTM

[matriv]

I'm considering to backport it to earlier versions too, more like a fix.
What would you think?

@costin what's your opinion on this ^ ?

[matriv]

@elasticmachine run elasticsearch-ci/packaging-sample