ci: Add CLA assistant

.github/workflows/EttusCLA.md

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+
+[Check dis out](https://files.ettus.com/licenses/Ettus_CLA.pdf)

.github/workflows/cla.yml

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+name: "CLA Assistant"
+on:
+  issue_comment:
+    types: [created]
+  pull_request_target:
+    types: [opened,closed,synchronize]
+
+jobs:
+  CLAssistant:
+    runs-on: ubuntu-latest
+    steps:
+      - name: "CLA Assistant"
+        if: (github.event.comment.body == 'recheck' || github.event.comment.body == 'I have read the CLA Document and I hereby sign the CLA') || github.event_name == 'pull_request_target'
+        # Beta Release
+        uses: cla-assistant/github-action@v2.1.3-beta
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          # the below token should have repo scope and must be manually added by you in the repository's secret
+          PERSONAL_ACCESS_TOKEN : ${{ secrets.CLA_ASSISTANT_UHD }}
+        with:
+          path-to-signatures: 'signatures/version1/cla.json'
+          path-to-document: 'https://github.com/EttusResearch/blob/master/uhd/.github/workflows/blob/master/EttusCLA.md'
+          # branch should not be protected
+          branch: 'main'
+          allowlist: user1,bot*
+
+         #below are the optional inputs - If the optional inputs are not given, then default values will be taken
+          remote-organization-name: EttusResearch
+          remote-repository-name: cla-signees
+          #create-file-commit-message: 'For example: Creating file for storing CLA Signatures'
+          #signed-commit-message: 'For example: $contributorName has signed the CLA in #$pullRequestNo'
+          #custom-notsigned-prcomment: 'pull request comment with Introductory message to ask new contributors to sign'
+          #custom-pr-sign-comment: 'The signature to be committed in order to sign the CLA'
+          #custom-allsigned-prcomment: 'pull request comment when all contributors has signed, defaults to **CLA Assistant Lite bot** All Contributors have signed the CLA.'
+          #lock-pullrequest-aftermerge: false - if you don't want this bot to automatically lock the pull request after merging (default - true)
+          #use-dco-flag: true - If you are using DCO instead of CLA
+