[roll third_party/pigweed] pw_crypto: Add Micro-ECC backed ECDSA

Adds Micro-ECC as a backend for ECDSA. Since Micro-ECC contains only a
single uECC.c, it is embedded in pw_crypto directly (as recommended by
the project's README.md).

Micro-ECC is also designated as the default ECDSA backend, because its
implementation is the smallest (~4KiB).

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/54382

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: df38a7de0dd6fa4..d502ff16a04d8f4
Roller-URL: https://ci.chromium.org/b/8838042110716142145
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8838042110716142145
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I2935364c17bb1df84c6208ab0dc70e08f633bcb4
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/58040
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index df38a7d..d502ff1 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit df38a7de0dd6fa4ee3939748d8ed6cec29475e4a
+Subproject commit d502ff16a04d8f4eb0e10a06ac7081c017a7699c