tokio-epoll-uring: fallback to std-fs if not available & not explicitly requested

[problame]

fixes #7116

Changes:

• refactor PageServerConfigBuilder: support not-set values
• implement runtime feature test
• use runtime feature test to determine virtual_file_io_engine if not explicitly configured in the config
• log the effective engine at startup
• drive-by: improve assertion messages in test_pageserver_init_node_id

This needed a tiny bit of tokio-epoll-uring work, hence bumping it.
Changelog:

    git log --no-decorate --oneline --reverse 868d2c42b5d54ca82fead6e8f2f233b69a540d3e..342ddd197a060a8354e8f11f4d12994419fff939
    c7a74c6 Bump mio from 0.8.8 to 0.8.11
    4df3466 Bump mio from 0.8.8 to 0.8.11 (#47)
    342ddd1 lifecycle: expose `LaunchResult` enum (#49)

[github-actions]

2688 tests run: 2563 passed, 0 failed, 125 skipped (full report)

---

Code coverage* (full report)

• functions: 28.4% (7069 of 24849 functions)
• lines: 47.0% (43412 of 92417 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
3db15cd at 2024-03-15T17:51:16.155Z :recycle:}

[arpad-m]

ok except typo

applied fix

[arpad-m]

legit clippy failures:

  error: an implementation of `From` is preferred since it gives you `Into<_>` for free where the reverse isn't true
     --> pageserver/src/virtual_file/io_engine.rs:272:1
      |
  272 | impl Into<IoEngineKind> for FeatureTestResult {
      | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      |
      = help: `impl From<Local> for Foreign` is allowed by the orphan rules, for more information see
              https://doc.rust-lang.org/reference/items/implementations.html#trait-implementation-coherence
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#from_over_into
      = note: `-D clippy::from-over-into` implied by `-D warnings`
      = help: to override `-D warnings` add `#[allow(clippy::from_over_into)]`
  help: replace the `Into` implementation with `From<virtual_file::io_engine::FeatureTestResult>`
      |
  272 ~ impl From<FeatureTestResult> for IoEngineKind {
  273 ~     fn from(val: FeatureTestResult) -> Self {
  274 ~         match val {
      |
  
  error: unneeded late initialization
     --> pageserver/src/virtual_file/io_engine.rs:305:17
      |
  305 |                 let remark;
      |                 ^^^^^^^^^^^
      |
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_late_init
      = note: `-D clippy::needless-late-init` implied by `-D warnings`
      = help: to override `-D warnings` add `#[allow(clippy::needless_late_init)]`
  help: declare `remark` here
      |
  306 |                 let remark = match e.raw_os_error() {
      |                 ++++++++++++
  help: remove the assignments from the `match` arms
      |
  308 ~                         "creating tokio-epoll-uring failed with non-OS error: {e}"
  309 |                     }
  310 |                     Some(nix::libc::EFAULT) => {
  311 ~                         "tokio-epoll-uring crate produces EFAULT?"
  312 |                     }
  313 |                     Some(nix::libc::EPERM) => {
  314 ~                         "creating io_uring fails with EPERM"
      |
  help: add a semicolon after the `match` expression
      |
  321 |                 };
      |                  +