Proposal: add Database Link to span context

## Description of the issue

In https://github.com/elastic/apm-agent-java/pull/696 support is being added for extracting the [Database Link](https://docs.oracle.com/cd/B28359_01/server.111/b28310/ds_concepts002.htm#ADMIN12083) from queries. This is an Oracle database feature for linking multiple databases to form a single logical database.

Queries across these databases may have different performance characteristics, since they may be geographically distributed, on different hardware, or under different load. As such it may be useful to know which remote database a query is operating on.

## Proposal

Add the following field to span context:

Intake API field | Elasticsearch field
-- | --
context.db.link (string, optional) | span.db.link (keyword)

Initially this field would not be displayed in the APM UI, as the link is already visible in the statement. It would be indexed separately for custom visualisations, breaking down metrics by destination database. (See https://github.com/elastic/apm-agent-java/pull/696#issuecomment-507132691)

@wolframhaussig @elastic/apm-agent-devs @elastic/apm-server please take a look and comment if you have any concerns.