Fix script bundle creation and improve PEM/Kelvin heap dump collection for large clusters

[ddelnano]

Summary: Fix script bundle creation and improve PEM/Kelvin heap dump collection for large clusters

The src/pxl_scripts/px/collect_heap_dumps.pxl was added at the root of a top level script directory. Since this script is largely used adhoc, this went unnoticed until I worked on #2245 and #2246. This moves the script into a proper directory structure and updates it to accommodate very large clusters (see relevant issue for more details).

Relevant Issues: #2245

Type of change: /kind bug

Test Plan: Verified pxl script runs in the UI and that bundle creation works again

# Fails on main

(main) $ make bundle-oss.json
px create-bundle --search_path /home/ddelnano/code/pixie-extra/src/pxl_scripts --base bpftrace --base px --base pxbeta --base sotw -o /home/ddelnano/code/pixie-extra/src/pxl_scripts/bundle-oss.json
Pixie CLI
FATA[0000] Failed to create bundle                       error="open /home/ddelnano/code/pixie-extra/src/pxl_scripts/px/manifest.yaml: no such file or directory"
make: *** [Makefile:28: bundle-oss.json] Error 1

# Succeeds with this change

(ddelnano/fix-script-bundle-improve-heap-dump-scripts) $ make bundle-oss.json
px create-bundle --search_path /home/ddelnano/code/pixie-extra/src/pxl_scripts --base bpftrace --base px --base pxbeta --base sotw -o /home/ddelnano/code/pixie-extra/src/pxl_scripts/bundle-oss.json
Pixie CLI

[ddelnano]

The linting issue will be fixed when #1955 is merged.