Add pvget servlet

[wanglin86769]

At one workshop section hosted by @shroffk at the Spring 2025 EPICS Collaboration Meeting this month, it seemed that a few developers were interested in adding pvget REST API to pvws, and it is the reason why this PR is created.

The implementation of the servlet is as follows,

1. It asynchronously reads a value from the given PV(s) with retry logic.
2. Single PV value can be fetched via query parameter "?pv=pvname" and JSON object will be returned.
3. Multiple PV values can be fetched together via query parameter "?pv=pvname1,pvname2,pvname3" and JSON array will be returned.
4. For simplicity, raw data instead of base64 encoding are returned for array value.

[wanglin86769]

Thanks for the comments, previously simulated PVs were not taken into account.

Now the following modifications have been made,

1. Pass in a JSON array of PV names instead of splitting by commas
2. Add a timeout for task execution
3. Break the retrying loop only if asyncRead() returns non-null value, but the behavior of asyncRead() for simulated PVs looks weird to me and I have created an issue in the Phoebus repo
   org.phoebus.pv.PV.asyncRead() throws exception for real PVs but returns null for simulated PVs when connection not established ControlSystemStudio/phoebus#3385

[kasemir]

You should not look for a non-null value but use PV.isDisconnected to check the value.
See more elaborate comments in ControlSystemStudio/phoebus#3385

[kasemir]

Are you creating a new thread pool for 100 threads each time the servlet is called?!?

[kasemir]

See also https://github.com/ControlSystemStudio/phoebus/blob/20dd0cce58b3cd9e52dda92ec02affb80dba51b1/core/pv/src/test/java/org/phoebus/pv/ReactivePVTest.java#L57 for example that creates PV, gets single value, releases.

That will most efficiently get you the current value, using the value we received last for a known PV.