Provide an option to configure the url pattern for RUM data

To address the [high url cardinality of RUM data](https://github.com/elastic/apm-agent-rum-js/issues/56) we decided as a first step to provide a configuration for the users to provide a url pattern that will help us group transactions based on the page url.

This config could accept a regex or another syntax for defining the url pattern.

Furthermore, this option could be fetched from central config. This has a few advantages, for example it lets both Kibana and potentially the RUM agent to consider this pattern on grouping and sampling of transactions. As well as providing easy and scalable configuration for the users

For more context [see this comment](https://github.com/elastic/apm-agent-rum-js/issues/56#issuecomment-641913346)

cc @axw