RFR: 6656: Allow capturing field values with path syntax

[Kangcheng Xu]

This patch implements [JMC-6656: Allow capturing field values with path syntax](https://bugs.openjdk.java.net/browse/JMC-6656).

In the xml configuration, a field capture looks like:
<field>
  <name>a field value</name>
  <description>a field value capture with a path syntax</description>
  <expression>this.field.prop</expression>
</field>
See `org/openjdk/jmc/agent/test/jfrprobes_template.xml` for more examples.

There are currently two limitations to pay attention to:

1. Instrumentation point cannot be in synthesized classes:
Instrumented classes are first loaded by obtaining the bytecode and inspected reflectively to resolve typing information. This fails if the class load is unable to provide the bytecode when `ClassLoader.getResouce()` is called. The cases where it might fail are unlikely to be for classes like proxies or auxiliaries for Java frameworks that have no visible equivalent.

2. Instrumentation is unable to access nestmates' private fields:
Before [nest-base access control](https://openjdk.java.net/jeps/181) was introduced in Java 11, accessing nestmates' private fields is, while lexically correct, forbidden by the JVM. The Java compiler works around by inserting synthetic getters/setters. However, it's not for a BCI agent since changing the class structure is not allowed during class transformation.

Please let me know your thoughts. Thank you very much!

-------------

Commits:
 - caaa7e7e: resolve conflicts after merging from master
 - 3383c080: Merge branch 'master' into agent-path-expression
 - a3e0c61e: error on private member access between nestmates
 - 5c19b765: add test cases for field capture feature
 - e05ea8bf: support legacy JFR api
 - 1019ca0d: change indentations to using tabs. add license headers
 - 1c7b0ac6: rename "watch" to "field"
 - e93a31e2: refactor error handling. cache resolved reference chain
 - 92b92502: add a QualifiedThisReference only when making qualified .this or .super
 - db3d6266: WIP: refactor
 - 040a42d8: add final modifiers
 - a803e7e7: remove unnecessary null checks
 - af33e32f: implemented path-syntax evaluation
 - 4cc7b9ea: WIP: add check for access and static context. improves error messages
 - 036b324e: WIP: state machine for parsing expression
 - 3f2f4023: WIP: syntax parsing with state machine
 - b58a44e4: WIP: implement implicit upwards reference in a nest
 - ad7d64f6: WIP: enforce access checks on reference chains
 - 5e46ed6f: WIP: avoid loading instrumentation pending classes with parent loader
 - 1964111f: WIP: fix loading "this"
 - 74624291: WIP: BCI agent path-syntax evaluation

Changes: https://git.openjdk.java.net/jmc/pull/20/files
 Webrev: https://webrevs.openjdk.java.net/jmc/20/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6656
  Stats: 2245 lines in 23 files changed: 2153 ins; 1 del; 91 mod
  Patch: https://git.openjdk.java.net/jmc/pull/20.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/20/head:pull/20

PR: https://git.openjdk.java.net/jmc/pull/20