RFR: 7033: Allow overriding the id (name) of the event type

[Gunnar Morling]

@thegreystone et al., this PR proposes two changes to the names of generated event types and their fields:

* An explicit event type name is derived from the name given in the configuration, instead of using the class name; this is nicer to work with e.g. when enabling/disabling event types for recordings
* the "field" prefix is removed from event type field names; when examining e.g. `RecordedEvent` instances, names are nicer to look at that way IMO

Could you log an issue for this change if you agree it's desirable? Thanks! I ran into these issues while using JMC Agent for [tracking SQL statements](https://twitter.com/gunnarmorling/status/1333152245463064579); note the odd "fieldSQLQuery" name marked on the left; the event type name is already adjusted in this screenshot as per this PR here.

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Commit messages:
 - Nicer names for event types and fields

Changes: https://git.openjdk.java.net/jmc/pull/170/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=170&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7033
  Stats: 20 lines in 2 files changed: 9 ins; 5 del; 6 mod
  Patch: https://git.openjdk.java.net/jmc/pull/170.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/170/head:pull/170

PR: https://git.openjdk.java.net/jmc/pull/170