Improve log group search

[idastambuk]

Log groups selector only fetches 50 log groups so we have a search field that users can search specific log groups by. Right now it searches by "log group prefix"

This is stored as logGroupPattern in the query, but replaced with log group prefix here:

if req.LogGroupNamePattern != nil {
			input.LogGroupNamePrefix = req.LogGroupNamePattern
		}

grafana-cloudwatch-datasource/pkg/cloudwatch/services/log_groups.go

Line 31 in ccf1f1b

if req.LogGroupNamePattern != nil {

which is a bit strange. Is this a limitation of the API? If not, this seems like something we should fix, as searching by pattern is more flexible than searching by prefix, since it searches by "contains" instead of "beginsWith" (like prefix does)