Add config variable for timeline offloading (#9421)

Adds a configuration variable for timeline offloading support. The added
pageserver-global config option controls whether the pageserver
automatically offloads timelines during compaction.

Therefore, already offloaded timelines are not affected by this, nor is
the manual testing endpoint.

This allows the rollout of timeline offloading to be driven by the
storage team.

Part of #8088

libs/pageserver_api/src/config.rs

@@ -102,6 +102,7 @@ pub struct ConfigToml {
     pub ingest_batch_size: u64,
     pub max_vectored_read_bytes: MaxVectoredReadBytes,
     pub image_compression: ImageCompressionAlgorithm,
+    pub timeline_offloading: bool,
     pub ephemeral_bytes_per_memory_kb: usize,
     pub l0_flush: Option<crate::models::L0FlushConfig>,
     pub virtual_file_io_mode: Option<crate::models::virtual_file::IoMode>,
@@ -385,6 +386,7 @@ impl Default for ConfigToml {
                 NonZeroUsize::new(DEFAULT_MAX_VECTORED_READ_BYTES).unwrap(),
             )),
             image_compression: (DEFAULT_IMAGE_COMPRESSION),
+            timeline_offloading: false,
             ephemeral_bytes_per_memory_kb: (DEFAULT_EPHEMERAL_BYTES_PER_MEMORY_KB),
             l0_flush: None,
             virtual_file_io_mode: None,