feat(rfc): add RFC: reader_options

core/src/docs/rfcs/0000_reader_with_options.md

@@ -0,0 +1,89 @@
+- Proposal Name: `reader_with_options`
+- Start Date: 2024-12-31
+- RFC PR: [apache/opendal#0000](https://github.com/apache/opendal/pull/0000)
+- Tracking Issue: [apache/opendal#0000](https://github.com/apache/opendal/issues/0000)
+
+# Summary
+
+Add `if_modified_since` and `if_unmodified_since` options to OpenDAL's `reader_with` API.
+
+# Motivation
+
+OpenDAL currently supports conditional `reader_with` operations based only on `version`. However, many storage services also 
+support conditional operations based on modification time.
+
+Adding these options will:
+
+- Provide more granular control over read operations.
+- Align OpenDAL with features provided by modern storage services, meeting broader use cases.
+
+# Guide-level explanation
+
+Two new options will be added to the `reader_with` API:
+
+## `if_modified_since`
+
+Return the content if it has been modified since the specified time; otherwise, 
+an error kind `ErrorKind::ConditionNotMatch` will be returned:
+
+```rust
+use chrono::{Duration, Utc};
+
+let last_check = Utc::now() - Duration::seconds(3600); // 1 hour ago
+let reader = op.reader_with("path/to/file")
+    .if_modified_since(last_check)
+    .await?;
+```
+
+
+## `if_unmodified_since` 
+
+Return the content if it has NOT been modified since the specified time; otherwise, 
+an error kind `ErrorKind::ConditionNotMatch` will be returned:
+
+```rust
+use chrono::{Duration, Utc};
+
+let timestamp = Utc::now() - Duration::seconds(86400); // 24 hours ago
+let reader = op.reader_with("path/to/file")
+    .if_unmodified_since(timestamp)
+    .await?;
+```
+
+
+# Reference-level explanation
+
+The main implementation will include:
+
+1. Add new fields(`if_modified_since`, `if_unmodified_since`) to `OpRead`.
+
+2. Add new capability flags:
+```rust
+pub struct Capability {
+    // ... other fields
+    pub read_with_if_modified_since: bool,
+    pub read_with_if_unmodified_since: bool,
+}
+```
+
+3. implement `if_modified_since`, `if_unmodified_since` for the underlying storage service.
+
+# Drawbacks
+
+- Add complexity to the API
+
+# Rationale and alternatives
+
+- Follows existing OpenDAL patterns for conditional operations
+
+# Prior art
+
+None
+
+# Unresolved questions
+
+None
+
+# Future possibilities
+
+None