Escape non-alphanumeric fields for EQL

[rw-access]

We need to add an escape sequence for fields that don't follow the typical IDENTIFIER regex: [_A-Za-z0-9][_A-Za-z0-9]*

One thought, and currently in the #49658 PR is to use back ticks ` around special fields. I believe this is how MS SQL works, and we currently check for that syntax within the SQL plugin but mark it as unsupported:

elasticsearch/x-pack/plugin/sql/src/main/java/org/elasticsearch/xpack/sql/parser/SqlParser.java

Lines 161 to 169 in ec1ac0d

	@Override
	public void exitBackQuotedIdentifier(SqlBaseParser.BackQuotedIdentifierContext context) {
	Token token = context.BACKQUOTED_IDENTIFIER().getSymbol();
	throw new ParsingException(
	"backquoted identifiers not supported; please use double quotes instead",
	null,
	token.getLine(),
	token.getCharPositionInLine());
	}

We should decide on an intuitive syntax that doesn't compete with single and double quotes.

[elasticmachine]

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

[costin]

I've added the discuss label so we can tackle it on our next meeting.

[rw-access]

We decided in the 2020/03/19 weekly sync to stick with backticks. Closing.

[colings86]

@rw-access I'm not sure you meant to close this? We should keep the issue open until we have made and merged the change/enhancement/feature.