Adding more config options to central config for backend agents

[felixbarny]

Let's add another batch of options, following up on #213.
The criteria is: important options you'd regularly want to change on-the-fly.

Option	Alignment status
sanitize_field_names	Not available for Node.js. Semantics in Python and Ruby are slightly different #334
transaction_ignore_urls	Aligned via #333 (7.10)
span_min_duration	Only available for Java. Spec PR: #314
log_level	Not available for python, values not aligned #332
transaction_max_spans	Already in central config

For reference, here's the table of config options for all agents: https://docs.google.com/spreadsheets/d/1JJjZotapacA3FkHc2sv_0wiChILi3uKnkwLTjtBmxwU

This is where the config options are defined in Kibana: https://github.com/elastic/kibana/blob/master/x-pack/plugins/apm/common/agent_configuration/setting_definitions/general_settings.ts

I'll create sub-issues for the options that are not aligned yet.

Open questions

• Does Kibana indicate which agent version is required to change a setting at runtime?
• Milestone/prioritization. This is not entirely clear yet but let's look at it as stretch goals for 7.10 for the time being