SQL: allow spaces in escape sequences

[bpintea]

Elasticsearch/SQL now supports JDBC/ODBC escape sequences at server level.
However, the current parser requires the { escape begin character to be followed by the escape discriminator with no white space allowed in-between. Example: FUNCTION_ESC: '{FN';.
The BNF specification suggests white spaces would be allowed (maybe an ABNF would have made that explicit) and in support for that, other DBs supporting these escape sequences at server level do allow for spaces too (like MSSQL, DB2, MariaDB/MySQL).
Furhermore, also some applications - like PowerBI - generate escape sequences with spaces:
select { fn name(params) } etc.

[elasticmachine]

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

[bpintea]

Alternatively, the statements spacing should to be adjusted in the respective drivers.

[matriv]

master : 08a22d0
7.x : e698e68
7.4 : aaea48d
6.8 : c2583e5