Add spiffe support proposal

proposals/0080-spiffe-support.md

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+## SPIFFE Support
+
+* **Owners:**
+  * `kfox1111`
+
+* **Implementation Status:** `Partially implemented`
+
+* **Related Issues and PRs:**
+  * https://github.com/prometheus/exporter-toolkit/issues/259
+  * https://github.com/prometheus/exporter-toolkit/pull/387
+
+> TL;DR: Flip a switch and exporters can mTLS with SPIFFE based identities
+
+## Why
+
+TLS is hard and manual. mTLS is even harder.
+
+SPIFFE and its corresponding reference implementation SPIRE makets it very easy to get fresh certificates in a fully automatic way.
+Connections between prometheus and exporters can be two way validated and encrypted.
+
+### Pitfalls of the current solution
+
+The main issue right now is the go-spiffe library pulls in otel and that pulls in a lot of dependencies. They compile out, but it looks bad.
+
+## Goals
+
+Support easy configuration of prometheus and exporters
+
+### Audience
+
+If not clear, the target audience that this change relates to.
+
+## Non-Goals
+
+* Move old designs to the new format.
+* Not doing X,Y,Z.
+
+## How
+
+We add SPIFFE support to the exporter-toolkit and prometheus scrape configuration
+
+## Alternatives
+
+Today you can wrap every exporter in a SPIFFE supporting proxy and put a proxy inbetween prometheus and the exporter too. It's painful. The idea is security should be easy.
+
+## Action Plan
+
+The tasks to do in order to migrate to the new idea.
+
+* [ ] Implement support in the exporter-toolkit
+* [ ] Implement support in prometheus