feat: in-memory pubsub component

[zach030]

What this PR does:
Add in-memory pubsub component and demo client

Which issue(s) this PR fixes:

Fixes #67

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov]

Codecov Report

Merging #329 (1a22d40) into main (fd12855) will not change coverage.
The diff coverage is n/a.

❗ Current head 1a22d40 differs from pull request most recent head 4848d3a. Consider uploading reports for the commit 4848d3a to get more accurate results

@@           Coverage Diff           @@
##             main     #329   +/-   ##
=======================================
  Coverage   56.90%   56.90%           
=======================================
  Files          48       48           
  Lines        2100     2100           
=======================================
  Hits         1195     1195           
  Misses        774      774           
  Partials      131      131           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fd12855...4848d3a. Read the comment docs.

[seeflood]

lgtm

[seeflood]

Thanks for your contribution!
I will try to contribute the in-memory state store component to dapr